Hi Bink,
My daughter (6 years old) is the tallest of her peers, both boys and girls, and she's vegan since birth. Annoyingly, she doesn't eat particularly varied, as she's terribly picky, though apparently that is common with younger children. I do think she gets enough calories at least, as she has a ferocious appetite.
The best thing you can do is to learn as much as you can about nutrition and cooking. Knowledge is power! Aim to learn a little more every day until you're starting to feel confident. I've written up some general advice below about nutrition. Please don't feel like you have to learn and understand everything right away. You'll have time to learn, don't worry! There are a lot of places online where you can learn more.
Rule number 1: You should ideally eat a variety of foods, and you also need to eat a sufficient amount of calories.
You should also make sure you eat protein-rich foods such as that found in legumes and legume products: beans, lentils, chickpeas, peanuts, tofu, peanut butter, refried beans, hummus, and a bit in peas. While there are also proteins in grains and grain products (rice, wheat, barley, oats, pasta, rice noodles, bread), it's the kind of protein found in legumes that is the hardest to get, so it makes sense to focus on that. Obviously, eggs also has good proteins, though you may want to limit your consumption of eggs due to the cholesterol.
Healthy fats are good for you. Sprinkle your salads with rapeseed oil. Seeds and nuts also has healthy fats, I believe.
You should also make sure you get enough calcium. As a vegan, my main source of calcium is fortified plant milks and yogurts. Vegetarians will also find calcium in animal milks and yogurts. Don't be afraid of fortified products. Calcium is super important, and it's not so easy to get enough.
A good rule of thumb is to have some kind of green side salad daily, maybe with your lunch or dinner. This salad could also include bell pepper.
As for vitamins, the main ones to remember are vitamin B12 and vitamin D. These are not found naturally in plant foods, but are found in small amounts in dairy products and eggs. Some foods are fortified with them as well. The best source is vitamin supplements. You can probably find such supplements in a local health food shop, or you can buy them online. The one I use is called Veg1 (it has B12, D, and some minerals all in one).
Vitamin D is needed to absorb calcium, but it also serves many other purposes. Vitamin B12 is important for protein metabolism. It helps in the formation of red blood cells and in the maintenance of the central nervous system. B12 deficiency can cause irreversible damage to your central nervous system.
There's a lot to learn, but you've already taken the most important step: making the decision to go vegetarian! Most people don't get that far. Also, most people (meat eaters in particular) know very little about nutrition, so you'll soon be way ahead of the crowd